There are various known methods for obtaining direct positive images by means of a diffusion transfer system. Thus, methods in which so-called direct action color materials, which are color materials with which a direct positive image is obtained, are used include methods in which dye developers are used, as disclosed, for example, in U.S. Pat. No. 2,983,606, and systems in which silver ion dye releasing agents (thiazolidine type color materials) are used, as disclosed, for example, in U.S. Pat. No. 4,468,451. Furthermore, system in which compounds which release diffusible dyes by means of an intramolecular nucleophilic substitution reaction under alkaline conditions are used have been disclosed, for example, in JP-A-51-63618 and JP-B-57-22099 (the terms "JP-A" and "JP-B" as used herein signify an "unexamined published Japanese patent application" and an "examined Japanese patent publication", respectively), and systems in which compounds which release diffusible dyes on reduction with a reducing agent which has not been consumed in the process of silver development are used have been disclosed, for example, in JP-A-53-110827 and JP-A-54-130927.
However, with all of these systems the development of silver halide and the dye releasing reaction occur at the same time, in parallel, and so there is a common disadvantage in that images with good discrimination cannot be obtained unless the rates of the two reactions are controlled very precisely.
On the other hand, methods of obtaining positive images using negative action color materials include systems in which a dye forming coupler is combined with dissolution type physical development as disclosed, for example, in British Pat. No. 1,330,524, and systems in which development inhibitor releasing couplers are used, as disclosed, for example, in JP-B-47-49611. However, with these systems the layer structure is complex since a mechanism by which the other silver salts in adjacent layers are developed in the unexposed parts, which is the parts where the development of silver halide does not proceed, is used, and control of the overall image forming reactions is inevitably very complicated.
In systems developed to overcome these problems, a negative type dye releasing agent is combined with a direct reversal emulsion, the dye releasing agent is oxidized in the unexposed parts, and a diffusible dye is released subsequently. Such systems have been described in detail in, for example, U.S. Pat. Nos. 3,928,312, 4,135,929 and 4,336,322. However, with this type of system a special reversal technique in which nucleating agents are combined with direct reversal emulsions is used and so these systems do not have general applicability. For example, as yet it has proved impossible to apply these techniques to systems in which development is carried out at a lower pH than that of the normal instant photographs or with dry heat development systems.